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Ann. Geophys., 25, 2641-2648, 2007
www.ann-geophys.net/25/2641/2007/
© European Geosciences Union 2007
Comparison of magnetic field observations of an average magnetic cloud with a simple force free model: the importance of field compression and expansion
R. P. Lepping1, T. W. Narock1,2, and H. Chen3
1NASA, Goddard Space Flight Center, Heliophysics Science Division, Greenbelt, MD 20771, USA
2Goddard Earth and Science Technology Center, University of Maryland, Baltimore County, MD, USA
3River Hill High School, Clarksville MD 21029, USA
Abstract. We investigate the ability of the cylindrically symmetric
force-free magnetic cloud (MC) fitting model of Lepping et al. (1990) to
faithfully reproduce actual magnetic field observations by examining two
quantities: (1) a difference angle, called β, i.e., the angle between the direction
of the observed magnetic field (Bobs) and the derived force
free model field (Bmod) and (2) the difference in magnitudes between the observed and
modeled fields, i.e., ΔB(=|Bobs|−|Bmod|),
and a normalized ΔB (i.e., ΔB/<B>) is also examined, all for a judiciously chosen set of 50 WIND
interplanetary MCs, based on quality considerations. These three quantities
are developed as a percent of MC duration and averaged over this set of MCs
to obtain average profiles. It is found that, although <ΔB> and
its normalize version are significantly enhanced (from a broad central
average value) early in an average MC (and to a lesser extent also late in
the MC), the angle <β> is small (less than 8°) and
approximately constant all throughout the MC. The field intensity enhancements are due mainly to
interaction of the MC with the surrounding solar wind plasma causing field
compression at front and rear. For example, for a typical MC, ΔB/<B> is:
0.21±0.27 very early in the MC, −0.11±0.10 at the center
(and −0.085±0.12 averaged over the full "central region," i.e.,
for 30% to 80% of duration), and 0.05±0.29 very late in the
MC, showing a double sign change as we travel from front to center to back,
in the MC. When individual MCs are examined we find that over 80% of them
possess field enhancements within several to many hours of the front
boundary, but only about 30% show such enhancements at their rear
portions. The enhancement of the MC's front field is also due to MC
expansion, but this is usually a lesser effect compared to compression. It
is expected that this compression is manifested as significant distortion to
the MC's cross-section from the ideal circle, first suggested by Crooker et
al. (1990), into a more elliptical/oval shape, as some global MC studies
seem to confirm (e.g., Riley and Crooker, 2004) and apparently also as
confirmed for local studies of MCs (e.g., Hidalgo et al., 2002;
Nieves-Chinchilla et al., 2005).
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